1. Field of the Invention
The present invention relates to a steel cord used for a molded rubber product such as a steel radial tire or a conveyor belt, and a steel radial tire using the steel cord.
2. Description of the Related Art
A steel cord is used as a reinforcing material for a molded rubber product such as a steel radial tire or a conveyor belt. A typical example of this steel cord is a steel cord of a two-layer strand (3+6) structure described in Jpn. Pat. Appln. KOKAI Publication No. 5-302283. Three steel wires are twisted to form a core strand, and then six steel wires are twisted on the surface of the core strand to form an outer strand to obtain the above (3+6) steel cord. Two-layer strand steel cords of this type are classified into an SZ twist type steel cord in which the twisting direction of the core strand is opposite to that of the outer strand, and an SS twist type steel cord in which the twisting direction of the core strand is the same as that of the outer strand.
A conventional (3+6) steel cord has a low productivity and a high manufacturing cost because it requires two twisting steps. Especially, in the manufacture using a buncher (double twister), the core strand of an SZ twist type steel cord is untwisted in the cross twisting step. The wires must, therefore, be twisted in advance at a pitch smaller than that of a final cord, and the productivity becomes low.
In the (3+6) steel cord, rubber tends not to fill up to the central portion of a three-wire twisted strand, and a space tends to form in the central portion of the core. When the central portion of the core has such space, internal corrosion and fretting wear tend to occur, thereby considerably shortening the service life of the steel cord.
In the steel radial tire, a steel cord must have a breaking strength equal to or higher than a given level. When the cross-sectional area of the steel cord is increased to obtain a desired breaking strength, the arrangement pitch of the steel cords in the calender process excessively increases, and a larger number of steel cords cannot be arranged on a rubber sheet.